1. Field of the Invention
The present invention relates to an ultrasonic probe device, particularly an ultrasonic probe device having a capacitive micromachined ultrasonic transducer, and a control method thereof.
2. Description of the Related Art
In recent years, as ultrasonic elements, much attention is paid to capacitive micromachined ultrasonic transducers (cMUTs). In general, each cMUT includes a lower electrode disposed on a substrate, an upper electrode disposed in a thin film which faces the lower electrode, and a cavity placed between the lower electrode and the upper electrode. When a voltage is applied between the lower electrode and the upper electrode, a capacitance changes between these electrodes, so that the thin film vibrates. By this vibration, the cMUT emits an ultrasonic wave. That is, the cMUT can transmit the ultrasonic wave. Furthermore, when the cMUT receives the ultrasonic wave, the thin film vibrates. By this vibration, charges electrized on the lower electrode and the upper electrode change. By detecting the change of the charges, the cMUT can detect the ultrasonic wave. That is, the cMUT can receive the ultrasonic wave. To allow the cMUT to transmit and receive the ultrasonic wave mentioned above, it is necessary to apply a DC bias voltage between the lower electrode and the upper electrode to beforehand electrize these electrodes. This fact is disclosed in, for example, Jpn. PCT National Publication No. 2005-510264.
An ultrasonic probe device is known in which an ultrasonic image is obtained by transmitting the ultrasonic wave from such a cMUT as mentioned and receiving its reflected wave in the cMUT. In the acquisition of the ultrasonic image by use of such a cMUT, a technique to improve a receiving efficiency of the ultrasonic wave is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-122344. In the ultrasonic wave which travels through a short distance, an amplitude at the reception is sufficiently large. In consideration of this fact, in the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2006-122344, the DC bias voltage to be applied is set to be low, when the ultrasonic wave reflected in the vicinity of the cMUT is received by the cMUT. The ultrasonic wave which travels through a long distance has small amplitude at the reception, and hence detection sensitivity is low when the DC bias voltage is low. In consideration of this fact, when the ultrasonic wave reflected at a position distant from the cMUT is received by the cMUT, the DC bias voltage to be applied is set to be high, to increase the sensitivity to the ultrasonic wave. Furthermore, in Jpn. Pat. Appln. KOKAI Publication No. 2006-122344, it is disclosed that the DC bias voltage is gradually increased to continuously receive the ultrasonic wave reflected at the near position and the ultrasonic wave reflected at the distant position.